1. Field of the Invention
The invention relates to a cellular system and a method of controlling power at which a signal is transmitted in a cellular system, and more particularly to a cellular system and a method for controlling communication at a base station during hand-over.
2. Description of the Related Art
In a cellular system operating in direct sequence-code division multiple access (DS-CDMA), since a common frequency band is assigned to a plurality of channels, radio waves for the plurality of channels and radio waves for other channels would interfere with each other. If those radio waves much interfere with each other, receipt quality in a desired radio wave would be deteriorated, resulting in disconnection in communication lines. Hence, the number of lines through which communication can be made with desired receipt quality being maintained, that is, line capacity is dependent on how much radio waves interfere with other.
A signal transmitted from a first mobile station located remoter from a base station is attenuated more rapidly in upward line with respect to power than a signal transmitted from a second mobile station located closer to a desired base station. Accordingly, if the first and second mobile stations would transmit signals in the same power, an interference wave would be greater in power than a desired radio wave, causing far-to-near effect which is a bar to making communication. Accordingly, power at which mobile stations transmit signals has to be controlled in upward line in such a manner that a base station receives signals from the mobile stations by even power.
On the other hand, such far-to-near effect as mentioned above is not caused in downward line. However, power at which a base station transmits a radio signal to a mobile station is controlled also in downward line such that the power could ensure minimum receipt quality in radio signals received in a mobile station, in order to reduce interference with other channels. Power at which a base station transmits a radio sill to a mobile station is controlled in a closed-loop control wherein measured receipt quality in received radio signals is compared to a target quality, and if the measured receipt quality is higher than the target quality, a base station transmits a transmission power control (TPC) signal to a mobile station to instruct the mobile station to reduce power at which a mobile station transmits a radio signal to a base station, whereas if the measured receipt quality is lower than the target quality, a base station transmits TPC signal to a mobile station to instruct the mobile station to increase power at which a mobile station transmits a radio signal to a base station.
A cellular system operating in code division multiple access (CDMA) usually uses soft hand-over technique in which if a mobile station approaches a boundary between cells, and hence, a transmission loss between a signal transmitted from a base station with which the mobile station is making communication, and a signal transmitted from an adjacent base station becomes equal to or greater than a predetermined threshold, the base station and the adjacent base station are both connected to the mobile station, and both the base stations transmit radio signals to the mobile station.
The soft handover technique ensures a mobile station to have enhanced receipt quality by virtue of diversity effect brought by transmission from a plurality of base stations even in the vicinity of a boundary among cells where transmission lose is usually high, and hence, receipt quality is likely to be deteriorated. Specifically, before a line through which a mobile station is making communication with a base station is disconnected, a line through which the mobile station can make communication with a next candidate of a base station is ensured, ensuring smooth hand-over without a pause.
However, downward line is accompanied with a problem that since a plurality of base stations transmit radio signals during soft hand-over, radio signals would interference with one another with the result of reduction in line capacity.
In order to solve the above-mentioned problem, Japanese Unexamined Patent Publication 11-69416 A has suggested a cellular system in which only selected base stations are allowed to transmit radio signals among base stations which are in hand-over condition, to thereby reduce interference in radio signals in downward line. Specifically, a base station transmits a pilot signal at predetermined power, and a mobile station measures receipt quality in pilot signals transmitted from base stations with which the mobile station establishes a line. The mobile station determines a base station transmitting a pilot signal having high receipt quality, as a base station through which the mobile station makes communication (hereinafter, such a base station is referred to as “transmission base station”), multiplexes a signal for designating a base station as a base station through which the mobile station makes communication, to a control signal, and transmits the thus multiplexed signal to a base station or base stations in upward line.
A base station having established a line with a mobile station transmits a radio signal to the mobile station through downward line only when the mobile station designates the base station as the transmission base station by transmitting a control signal to the base station, and a base station or base stations which receive(s) a designation signal by which the base station or base stations is(are) not designated as a transmission base station does(do) not transmit a radio signal to the mobile station. Thus, it would be possible to make only a base station presenting small transmission loss to make communication with a mobile station, ensuring reduction in interference in downward line and an increase in line capacity.
However, the above-mentioned conventional method of controlling power at which a radio signal is transmitted is accompanied with a problem that a base station might wrongly receive a designation signal by which the base station is designated as a transmission base station. In particular, if a transmission base station wrongly receives a designation signal by which the transmission base station is not designated, and hence, stops transmitting radio signals, all of he base stations having established a line with the mobile station might not transmit radio signals to the mobile station.
If such a case occurs, since a radio signal is not transmitted at all throughout an updating period in the transmission base station, it would not be possible to accomplish normal demodulation even by interleave or error correction in a mobile station. This increases possibility of receipt error, and hence, increases an error rate in a mobile station.
Accordingly, a signal-signal-receipt error rate which can be accomplished in downward line is restricted by a signal-signal-receipt error rate of a control signal in upward line. Hence, if a requested target signal-signal-receipt error rate is quite low, it might not be possible to accomplish such a low target signal-signal-receipt error rate.
In order to solve such a problem, it would be necessary to minimize a rate at which a transmission base station does not transmit radio signals at all. To this end, it would be necessary to render a signal-signal-receipt error rate of a control signal in a transmission base station as small as possible, for instance, by providing high power for transmitting a control signal by which a base station is designated as a transmission base station, or by providing a control signal with high redundancy.
However, the above-mentioned solution would cause another problem of reduction in a line capacity in upward line and a longer period for updating a transmission base station.
As one of solutions to the above-mentioned problem, a plurality of base stations may be kept selected as a transmission base station. In the solution, even if signal-receipt error occurs in one of the transmission base stations, a possibility of stopping transmitting radio signals in all base stations can be rendered small by carrying out transmission of radio signals in other transmission base stations.
However, this solution would cause another problem that interference in radio signals transmitted from a plurality of base stations is increased, and hence, a line capacity in downward line is reduced.
Japanese Unexamined Patent Publication No. 11-275624 A has suggested a mobile communication system in which at least one base station having transmission loss equal to or smaller than a predetermined threshold among base stations which are in soft hand-over condition with a mobile station is selected as a communication base station for making communication with the mobile station, and power at which base stations other than the selected communication base station transmits a radio signal is uniformly attenuated to a predetermined minimum power.
Japanese Unexamined Patent Publication No. 11-355204 A has suggested a CDMA mobile communication system including a plurality of base stations each of which is connected to a mobile terminal through radio line for making communication with the mobile terminal, and a base station controller which switches the base stations with which the mobile terminal makes communication, and controls power at which the mobile terminal transmits a radio signal. Each of the base stations includes first means for detecting signal-receipt quality of a signal transmitted from the mobile terminal, a first comparator which compares the signal-receipt quality detected by the first means to a predetermined threshold, second means for, when the signal-receipt quality is equal to or higher than the predetermined threshold, measuring a frame error rate higher than a first standard level in the signal-receipt quality, a second comparator which compares the frame error rate to a predetermined threshold, third means for determining target signal-receipt quality, based on the result of comparison carried out by the second comparator, a third comparator which compares the signal-receipt quality measured by the second means to the target signal-receipt quality determined by the third means, and fourth means for generating and transmitting a control signal for controlling power at which the mobile terminal transmits a radio signal, based on the result of comparison carried out by the third comparator.
Japanese Unexamined Patent Publication No. P2000-138633 A has suggested a method of controlling power at which a radio signal is transmitted, including the steps of comparing signal-receipt quality of a signal transmitted from an opposite station, controlling power at which the opposite station transmits a signal, based on the result of comparison, checking whether there is a frame error in the received signal, increasing a control target, if there is a frame error, and gradually decreasing a control target, if there is not a frame error.
Japanese Unexamined Patent Publication No. P2000-252917 A has suggested a method of controlling power at which a radio signal is transmitted, in a cellular system, including the steps of comparing SIR of a received signal in a receiver to a target SIR, and controlling power at which a transmitter transmits a signal, based on a difference between them and/or whether which is greater, characterized by the steps of detecting a frame error in a received signal, counting the number n(k) of frame errors in an immediately prior N(k) frame (1≦k≦K) for every M(k) frames or each time a frame error occurs, and increasing or decreasing a target SIR in accordance with the frame error numbers n(1) to n(K) alone or in combination.
International Publication WO97/50197 (PCT/JP97/02215) has suggested an apparatus of controlling power at which a signal is transmitted, to be equipped in one of a base station and a mobile station both of which constitutes a mobile communication system, including first means for measuring SIR of a received signal, second means for comparing SIR measured by the first means to a predetermined target SIR, third means for transmitting data about signal transmission power to an opposite station, based on the result of comparison carried out by the second means, fourth means for receiving and demodulating the above-mentioned data transmitted from the opposite station, fifth means for controlling power at which a signal is transmitted, based on the demodulated data, sixth means for measuring an error rate in received signal, and seventh means for varying the target SIR in accordance with the error rate measured by the sixth means.
However, the above-mentioned problem remains unsolved even in the above-mentioned Publications.